The present invention relates generally to hydromechanical and electro-hydro-mechanical systems commonly utilized for positioning mechanical objects (which complete systems will hereinafter be called "control systems").
Most control systems typically utilize a control valve for selectively controlling the application of pressurized hydraulic fluid to the input ports of power output transducer. The power output transducer is operable for positioning the mechanical objects in response to the pressurized hydraulic fluid acting thereon. In addition, control systems often include feedback elements which are used for establishing a "reference" position for the control valve in order to form a closed-loop servo system. Conventional control systems generally include a pumping apparatus for supplying the pressurized hydraulic fluid at either a substantially constant pressure (hereinafter called "constant pressure systems") or a substantially constant flow rate (hereinafter called "constant flow systems").
Constant pressure systems are the most common. The control valves in constant pressure systems are utilized to control hydraulic fluid flow (hereinafter "load flow") to the input ports of the power output transducers. If the control valve of a constant pressure system is zero-lapped (i.e. as defined in a book entitled HYDRAULIC SYSTEM ANALYSIS by George R. Keller) then only leakage amounts of hydraulic fluid will be consumed whenever the control valve is centered. Furthermore, if the constant pressure system includes a feedback element for establishing a reference position for the control valve, then the constant pressure system will achieve its final position, with respect to the reference position, substantially without error.
Two methods of supplying hydraulic fluid at a substantially constant pressure (hereinafter "supply pressure") to the control valve are commonly utilized in constant pressure control systems. The simplest method utilizes a full-flow constant displacement pump, which supplies the hydraulic fluid at a nominally constant output flow rate, in conjunction with a relief valve that is used for setting the desired supply pressure. The relief valve is adapted to bypass surplus hydraulic fluid (i.e. that quantity of hydraulic fluid not utilized by the control system's power output transducer) to a reservoir with a pressure drop that is substantially identical to the desired supply pressure. Such a full-flow pump assembly is fully described in U.S. Pat. No. 3,048,119 entitled Air-Cooled Pump Assembly For Hydraulic Fluid And The Like by C. W. Tydeman that issued on Aug. 7, 1962. As disclosed therein, such full-flow pump assemblies are extremely inefficient and must be capable of thermally dissipating the full output power of the pump.
A more efficient method of maintaining the desired supply pressure is to utilize a flow regulating pump assembly which includes a variable displacement pump. The volumetric output of the variable displacement pump is selectively regulated to match the load flow and thereby achieve the desired supply pressure. Thus, energy losses are minimized. However, since most control systems are subject to widely fluctuating load pressures and virtually all of the energy represented by the product of load flow and the difference between the supply pressure and load pressure is converted to thermal energy, even a control system incorporating such a flow-regulated pump assembly can be quite inefficient.
Typical examples of control systems that are subject to such widely fluctuating load pressures are fully described in U.S. Pat. No. 3,048,119 as well as U.S. Pat. No. 3,145,597 entitled Hydraulically Operated Tracer Assembly For Engine Lathes, by C. W. Tydeman which issued on Aug. 25, 1964. FIG. 11 of U.S. Pat. No. 3,145,597 shows a control valve in an open position while FIG. 8.12 of the book entitled HYDRAULIC SYSTEM ANALYSIS depicts a range of flow-pressure coefficients for a zero-lapped 4-way control valve (i.e. similar to that disclosed in U.S. Pat. No. 3,145,597). As shown in FIG. 8.12, load flow through such a control valve varies with respect to load pressure according to a square root law (i.e. with respect to the difference between the supply pressure and the load pressure) for any particular valve opening.
Thus, it is apparent that the valve opening must be altered in order to maintain constant load flow in the face of a varying load pressure. In the case of the tracer assembly described in U.S. Pat. No. 3,145,597, this means that deflection of the control valve will vary as a function of the cutting load imposed upon an associated cutting tool for any particular non-zero tracer velocity. As such, constant pressure systems have an error signal which suffers modulation as a function of control system load for any non-zero value of control system output velocity.
Constant flow systems are typically utilized for vehicular power steering systems. Control valves utilized in such constant flow control systems are severely under-lapped (i.e. having an open-center configuration) in order to provide passage of the hydraulic fluid without generating an objectionable parasitic pressure loss. This permits the utilization of simple constant displacement pumps with concomitantly minimized average power consumption. However, this also results in a control characteristic wherein valve deflection primarily regulates load pressure rather than load flow. Furthermore, the load pressure is generally highly nonlinear with respect to valve deflection. U.S. Pat. No. 4,460,016 entitled Rotary Servovalve by Haga et al, issued Jul. 17, 1984 discloses the various factors relating to these control characteristics. Thus, such control systems are typically subject to large position error even under relatively light steering loads.
These performance characteristics, associated with most conventional vehicular power steering systems, substantially conform to a type "0" servo system as defined in a book entitled FEEDBACK AND CONTROL SYSTEMS by Di Stefano III, Stubberud and Williams and published as one of Schaum's Outline Series in Engineering by McGraw-Hill. In general, type "0" servo systems are subject to a fixed value of steady state position error and a totally undefined velocity error when subject to steady state loads. Alternatively, it is desirable to configure a vehicular power steering systems as a type "1" servo system. This is because type "1" servo systems have zero position error and a fixed value of velocity error when subject to steady state loads. In any case, the result (with rotary valve equipped power steering systems) is often a feeling of "play" with concomitant "wander" of a host vehicle when the vehicle is subject to transient load conditions (i.e. such as intermittent side winds or uneven road surfaces).
For this reason some vehicular power steering systems incorporate mechanically interlocking over-ride mechanisms whereby direct coupled manual steering is engaged at light steering loads. However, even such vehicular power steering systems have degraded road feel when compared to manual steering systems. This is because of the parasitic drag associated with various components (i.e. hydraulic power cylinder, seals and the like) and a highly non-linear transition from manual steering to power assisted steering at a selected value of steering load.
Accordingly, the present invention includes various embodiments of control systems having a pumping apparatus which supplies pressurized hydraulic fluid at variable pressures that are selectably related to load pressure. In a first series of embodiments, any one of a series of pressure regulating valve assemblies is used to selectively by-pass excess hydraulic fluid flow from a pump. This is done in a manner which develops a supply pressure that nominally obeys the equation: EQU P.sub.S =K.sub.1 .vertline.P.sub.L .vertline.+.DELTA.P
where P.sub.S is the supply pressure, .vertline.P.sub.L .vertline. is the absolute value of load pressure, K.sub.1 is a selected proportionality factor (whose value is usually selected to be 1.0 or very slightly higher in order to compensate for the effects of system leakage and losses) and .DELTA.P is a desired minimum value of P.sub.S present at a zero value of P.sub.L. Thus, the pressure drop through the control valve is maintained at a nominally constant value equal to the difference between the supply pressure and the load pressure.
As will be described below, the nominally constant pressure drop across the control valve results in a more nearly constant, or "stiffer", control valve flow control characteristic with respect to changes in load pressure. In this regard it is similar to a positive feedback technique commonly called "bootstrapping" which is utilized to "stiffen" the output voltage of electronic amplifiers. Because of functional similarity between the bootstrap electronic amplification technique and the control systems to be described hereinbelow, control systems having the pumping apparatus supplying pressurized hydraulic fluid at variable pressures that are selectably related to load pressure will hereinafter to be called "bootstrap control systems". Moreover, bootstrap control systems which use pressure regulating valve assemblies to selectively by-pass excess hydraulic fluid are hereinafter referenced to as "by-pass bootstrap systems".
In a second series of embodiments, any one of the first series of pressure regulating valves assemblies, as modified for significantly lower flow, is utilized for selectively controlling the volumetric output of a variable displacement pumping apparatus, with these control systems hereinafter being called "regulated bootstrap systems". This is done in a manner whereby the supply pressure substantially obeys the above equation, and concomitantly, load flow is substantially matched by pump volumetric output.
By-pass and regulated bootstrap systems usually incorporate control valves that are zero lapped or over-lapped such that valve leakage is minimal. Since supply pressure is virtually always maintained at a minimum level necessary for control valve function in a bootstrap control system, efficiency is substantially improved when compared to a conventional constant pressure system. In fact, efficiencies in a regulated bootstrap system can even approach those commonly associated with pulse-width modulated electronic servo systems.
Since the difference between the supply pressure and the load pressure is nominally constant regardless of the load value (for positive loads), constant load flow can be substantially maintained without changing valve opening. Therefore, the value of the error signal is substantially a function of control system output velocity. Thus, in "bootstrap" systems the error signal does not suffer modulation in the face of changing control system load values. In the case of the tracer assembly cited above, this would mean that there would be virtually no supplemental deflection of its stylus as a function of cutting load if that tracer assembly were re-configured as a bootstrap system.
Constant pressure systems are often configured with more than one control valve-power output transducer sub-system. The bootstrap systems of the present invention can be configured in this manner as well. More specifically, multiple by-pass bootstrap systems can utilize one prime mover-pump assembly via incorporating a flow divider. Likewise, multiple regulated bootstrap systems can be implemented by utilizing one prime mover to drive a ganged assembly of variable displacement (i.e. vane type) pumps. In either case, separate hydraulic circuits are used for each control valve-power output transducer sub-system.
According to another feature of the present invention, a four-way control valve of the type preferably utilized in the aforementioned bootstrap system is enclosed. The four-way control valve features zero-lapped or slightly over-lapped control orifices (i.e. it is a closed-center four-way control valve). Because of its closed-center design, valve deflection of the four-way control valve is primarily determined by load flow rather than load pressure. A primary benefit gained thereby is nominally zero valve deflection for any static load, even when used in constant flow bootstrap systems. Thus, when such constant flow bootstrap systems are incorporated into closed-loop servo systems, the resulting performance substantially conforms to that of type "1" servo systems as mentioned above.
The control valve is generally a modified rotary valve and, more particularly, is normally configured as a progressive rotary valve wherein the valve rotor and/or sleeve slots are formed such that the orifice width increases as valve deflection increases. This results in a progressively increasing velocity gain factor for the four-way control valve which allows both precise fine control and rapid full input motions. This progressive design characteristic can be effected via forming either of the valve rotor and/or sleeve slots with a helix angle so that a triangular orifice area is generated concomitant with valve deflection. Alternately, triangular or trapezoidal imprinting of edges of straight valve rotor slots may be performed.
When the four-way control valve of the present invention is utilized in a vehicular power steering system, the primary tactile feedback is related to steering wheel rotational velocity as opposed to steering force. Such tactile feedback can be thought of as negative rate feedback. Looked at another way, this primary steering characteristic is actually a positive real value of steering impedance as determined by steering wheel torque divided by steering wheel velocity. This novel steering characteristic is desirable because of its fundamentally stable feel as opposed to a "spring-like" feel (i.e. a negative imaginary value of steering impedance) present with many rotary valve equipped power steering systems.
Accordingly, a first preferred power steering system described herein is a hydro-mechanical by-pass bootstrap system. Another preferred power steering system is a hydro-mechanical constant pressure system that is useful when it is desired to implement the power steering system with an existing constant pressure hydraulic fluid supply (i.e. such as ABS brakes, automatic ride control or miscellaneous hydraulically powered equipment).
Another preferred power steering system comprises an electrically powered vehicular power steering system wherein a regulated bootstrap system is utilized. In the electrically powered vehicular power steering system a controller provides an electrical power signal to a motor driven pump in order to selectively control volumetric hydraulic fluid output.
Other features, objects and advantages of the present invention will become readily apparent to one skilled in the art upon analysis of the following written description taken in conjunction with the accompanying drawings and appended claims.